It sounds like the unit has some interesting "features".

The speed sensor and backup sensor suggest to me that it is using some dead-reckoning in addition to the satellite signals. And probably has an electronic compass.

Usually this is so when you get in urban areas or tunnels where satellite coverage is poor, the system can still "calculate" where you are by direction of travel and speed. I am surprised however that if these things are not connected the unit doesn't work.

A GPS doesn't need these things to work. I guess the factory unit does based on the problems reported.

Glad to hear it worked out OK, and thanks for the learning experience about the system.